1. Field of the Invention
The present invention relates to signal processing apparatus for processing the output signals from a solid state image sensor, for example, a charge coupled device (CCD) image sensor, and more particularly to a signal processing circuit for performing defect correction and image enhancement on such output signals.
2. Discussion Related to the Problem
The use of solid state image sensors in television cameras, electronic still cameras, and film scanners for producing TV signals from pictures on film is well known. It is also know to be desirable to image enhance the output signals produced by such image sensors. Well known techniques for image enhancement include crispening, sometimes called unsharp masking, to accentuate the appearance of fine detail; and noise reduction, for example "coring" used to suppress the appearance of film grain in film scanners. In the manufacture of solid state image sensors, such as CCD image sensors, many devices are produced having relatively few defects, but relatively few devices are produced having no defects. It is well known that if a relatively few defects in such devices can be accepted, the manufacturing yield of useful devices increases dramatically. It has been found however, that when the output signal of a device having a few tolerable defects is subjected to image enhancement, such as crispening, the appearance of the defect becomes enhanced, rendering the appearance of the defect intolerable.
Several approaches have been suggested for correcting defects in the output signals of image sensors. In one such approach, the output signal of the image sensor is sampled by a sample-and-hold circuit. When a defect is present in the output signal, the sample-and-hold circuit is not clocked, thereby replacing the defective portion of the signal with an adjacent undefective portion. For an example of this type of defect correction, see U.S. Pat. No. 4,167,754 issued Sept. 11, 1979 to Nagumo et al. It is also known to correct defects in the output signals of image sensors by interpolating between adjacent non-defective portions of the signal to compute a replacement value for the defective portion.
It would appear to be desirable to implement defect correction prior to image enhancement of the output signal of a solid state image sensor to avoid enhancement of the defects in the signal. Unexpectedly, however, it has been found that under some circumstances, for example in the scanning of color negative film to produce a TV signal, the appearance of the image-enhanced, defect-corrected signal is even more objectionable than the appearance of an enhanced signal to which no defect correction has been applied.
The challenge faced by the inventor therefore, was to provide a signal processing method and apparatus for defect correction and image enhancement that avoided the problems noted above.